This invention relates to a digital wireless wideband RF communication system operating in periodic noise environments. Such a system is useful, for example, for communicating digital data at high data rates; e.g. rates greater than 1 Mega bits per second (Mbps).
With the advent of digital photography, the wireless transmission of digital images, for example between a portable transmitting device like a digital camera and a receiving device such as a personal computer or other image appliance such as a printer, has become a desirable feature. High data rate transmissions are desirable because digital images represent a large amount of data and short transmission times are needed. Short transmission times result in shorter wait times while an image is being transferred from a camera to a receiver and in reduced battery power consumption.
Although there are several useful communication bands available for such a digital communication system, the 2.4 to 2.5 GHz ISM Band is particularly attractive because the band is unlicensed and available internationally. However, a major problem exists with the use of this band for wireless communication. The band is allocated for microwave ovens and other devices, which as described below, generate a great deal of periodic noise in the band. Other communication bands like the 5.75 GHz band can also have periodic noise generators present. For the purpose of describing the present invention, references will only be made to periodic microwave noise as is present in the 2.4 to 2.5 GHz ISM band. It will be understood that the same principles can be used in the presence of any other kind of periodic noise in any communication band.
Radio frequency (RF) transmissions in the 2.4 to 2.5 GHz ISM Band have historically had to deal with the presence of man-made noise from microwave ovens that predominately operate in the center of this band at 2.45 GHz. The noise emanates from the oven by leakage through the enclosure of the oven. The leakage noise is periodic and has a radiated output power approximately 20 dB greater in strength than that allowed by the FCC for operation of Part 15 non spread spectrum radios. With over 200 million microwave ovens in use throughout the world, they are by far the greatest and most significant source of noise in this band. In addition microwave lighting and illumination systems are soon to be in use in the same band creating additional noise interference. Some examples of locations where it would be desirable to transmit images in the presence of periodic microwave noise are in the home (particularly in the kitchen), or in a supermarket or department store where a photo kiosk may be located near a microwave oven or lighting system.
One possible way to communicate in the presence of microwave noise is to use a spread spectrum communication technique. One type of spread spectrum technique spreads the signal over a band which is much larger than the bandwidth of the signal so that the narrow band noise from the noise source has a reduced effect on the demodulated signal to noise ratio. This technique however is relatively expensive to implement, significantly limits the data throughput (e.g. by a factor of 8 or more) and does not work well if the receiver is located very close to the noise source. Another way to communicate in the presence in the presence of microwave noise is described in U.S. Pat. No. 5,574,979, issued Nov. 12, 1996 to West, entitled xe2x80x9cPeriodic Interference Avoidance in a Wireless Radio Frequency Communication System.xe2x80x9d This patent demonstrates a potential solution by avoiding microwave oven periodic noise by sensing the periodicity of the AC power line main in which the oven is connected to. Unfortunately this technique does not work for periodic noise sources other than microwave ovens or in cases where multiple microwave noise sources on different phases exist. This technique also does not work in cases where the RF communication equipment is battery powered and no connection to an AC power main can be made, nor can the AC main E field radiation be received by the RF communication equipment. Furthermore this approach does not address the problem where one of the wireless communication stations can sense the noise, but the other wireless communication station cannot; when both stations are subjected to noise having different characteristics; or when three or more stations are trying to communicate and one or more are in a noise environment.
There is a need therefore for an improved means of digital communication in environments with periodic noise.
This need is met by a wireless communication system operating in the presence of periodic noise signals such as from a microwave oven, that includes at least two wireless communication stations with each station including a transceiver, each such transceiver. Each transceiver includes a detector for detecting periodic noise signals and producing a first signal indicating the presence of noise signal""s quiescent period and second signals indicating the end of quiescent periods of such periodic noise signal. A controller responsive to the first and second noise signals controls the transceiver to provide effective communication with an other transceiver which minimizes the noise effect of the periodic noise signal. The transceiver transmits information concerning the detected quiescent period for use by the other transceiver to determine when to transmit during such quiescent period.
Advantages of the present invention include:
1. When each station can tell the other station(s) when periodic microwave signals will prevent reception, the transmitting stations can wait to transmit during the proper time. While waiting, systems may be powered down to reduce energy consumption, reduce interference with other devices and more efficiently control data flow to and from the host CPU; and
2. If two stations have interference between them, either partially or completely overlapping in time, the stations can look for other available stations with which to communicate in a more efficient manner, or wait until the first station is more available.
This invention is less expensive than one that uses a master clock to assign a global time reference to all stations and their quiescent periods. Such a system requires expensive accurate internal time bases or access to a Global Positioning System time base such as used by Code Division Multiple Access cellular systems.